Skip to main content
Browse Subject Areas

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Effects of six pyrimidine analogs on the growth of Tetrahymena thermophila and their implications in pyrimidine metabolism

  • Zander Harpel ,

    Contributed equally to this work with: Zander Harpel, Wei-Jen Chang

    Roles Formal analysis, Investigation, Software, Writing – original draft

    Affiliation Department of Biology, Hamilton College, Clinton, NY, United States of America

  • Wei-Jen Chang ,

    Contributed equally to this work with: Zander Harpel, Wei-Jen Chang

    Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing – original draft (WJC); (RW)

    Affiliation Department of Biology, Hamilton College, Clinton, NY, United States of America

  • Jacob Circelli,

    Roles Data curation, Investigation

    Affiliation Department of Biology, Hamilton College, Clinton, NY, United States of America

  • Richard Chen,

    Roles Data curation, Investigation

    Affiliation College of Literature, Science, and Arts, University of Michigan, Ann Arbor, MI, United States of America

  • Ian Chang,

    Roles Data curation, Investigation

    Affiliation Clinton Senior High School, Clinton, NY, United States of America

  • Jason Rivera,

    Roles Data curation, Investigation

    Affiliation Clinton Senior High School, Clinton, NY, United States of America

  • Stephanie Wu,

    Roles Data curation, Investigation

    Affiliation Department of Biology, Hamilton College, Clinton, NY, United States of America

  • RongHan Wei

    Roles Conceptualization, Investigation, Methodology, Project administration, Supervision, Validation, Visualization, Writing – original draft (WJC); (RW)

    Affiliation Engineering Technology Research Center of Henan Province for MEMS Manufacturing and Applications, School of Mechanics and Safety Engineering, Zhengzhou University, Zhengzhou, Henan, China


Tetrahymena are ciliated protists that have been used to study the effects of toxic chemicals, including anticancer drugs. In this study, we tested the inhibitory effects of six pyrimidine analogs (5-fluorouracil, floxuridine, 5’-deoxy-5-fluorouridine, 5-fluorouridine, gemcitabine, and cytarabine) on wild-type CU428 and conditional mutant NP1 Tetrahymena thermophila at room temperature and the restrictive temperature (37°C) where NP1 does not form the oral apparatus. We found that phagocytosis was not required for pyrimidine analog entry and that all tested pyrimidine analogs inhibited growth except for cytarabine. IC50 values did not significantly differ between CU428 and NP1 for the same analog at either room temperature or 37°C. To investigate the mechanism of inhibition, we used two pyrimidine bases (uracil and thymine) and three nucleosides (uridine, thymidine, and 5-methyluridine) to determine whether the inhibitory effects from the pyrimidine analogs were reversible. We found that the inhibitory effects from 5-fluorouracil could be reversed by uracil and thymine, from floxuridine could be reversed by thymidine, and from 5’-deoxy-5-fluorouridine could be reversed by uracil. None of the tested nucleobases or nucleosides could reverse the inhibitory effects of gemcitabine or 5-fluorouridine. Our results suggest that the five pyrimidine analogs act on different sites to inhibit T. thermophila growth and that nucleobases and nucleosides are metabolized differently in Tetrahymena.


Tetrahymena thermophila are unicellular, ciliated protists whose function as a model eukaryote has contributed to the discovery of catalytic RNA, telomerase and telomere structure, and the first histone acetyltransferase, among numerous other contributions [13]. In addition to their important role in the lab, Tetrahymena species living in diverse natural freshwater habitats have been used to indicate levels of pollutants present in ecosystems and to examine chemical toxicities [4, 5], including classes of anticancer drugs [47]. Among these anticancer drugs, we are particularly interested in pyrimidine analogs [6, 8] as their ability to block the growth of Tetrahymena allows for the study of nucleotide uptake and metabolism in this species. Moreover, because Tetrahymena lacks de novo nucleotide synthesis pathways [9, 10] we may gain further insight into the metabolism of these compounds in this organism by manipulating the exogenous supply of nucleotides [11].

Because Tetrahymena cannot synthesize their own nucleotides de novo (review in [8]), they must obtain nucleobases, nucleosides, and nucleotides from food sources or their environment. Studies have shown that uracil, uridine, deoxyuridine, cytidine, deoxycytidine, CMP, dCMP, UMP, and dUMP could each effectively serve as the lone pyrimidine source to support the growth of T. pyriformis in defined medium [9, 1113]. These observations suggest that most cytidine and uridine derivatives, except for cytosine which is inert in supporting the growth of T. pyriformis [9, 11, 12], may be deaminated and aminated to form necessary pyrimidines. Moreover, comparisons of T. pyriformis doubling time in defined medium suggested that uracil and uridine are equally effective in supporting Tetrahymena growth [11].

In contrast to the effective pyrimidine sources mentioned earlier, methylated pyrimidines, such as thymine, thymidine, TMP, and 5-methyluridine could not function as the lone pyrimidine source, suggesting that T. pyriformis lacks a pathway to demethylate these thymidine derivatives [1113].

As a unicellular organism, how nutrients are obtained and enter Tetrahymena cells is also of great interest [14]. In addition to the distinct phagocytosis through their oral apparatus, Tetrahymena has at least four other pathways of endocytic uptake through their peripheral membrane [15]. While phagocytosis plays an important role in obtaining nutrients in Tetrahymena, there are also indications that some amino acids, nucleobases, and nucleosides may be absorbed through their peripheral membrane system or endocytosis [1618]. Rasmussen first reported that at a growth condition where food vacuoles formed slowly, the doubling time of T. pyriformis could be shortened by supplementing the sterile filtered proteose peptone medium with high concentrations of nucleotides and glucose [18]. Using the conditional mutant NP1, a strain that does not form food vacuoles at the restrictive temperature (37°C), Rasmussen and Orias showed that at 37°C, ‘mouthless’ NP1 could multiply quickly (every 3.5 hours) in a two percent proteose peptone medium supplemented with high concentrations of vitamins and heavy metal salts [17]. Their finding further indicates that nucleobases and their derivatives, which are essential for Tetrahymena growth, could enter Tetrahymena cells through the peripheral membrane system. Freeman and Moner fed T. pyriformis GL-7 with [3H]uridine in a short period of time before a new food vacuole could be formed and quantified the amount of radioactive uridine present in different forms in the cell [16]. They concluded that uridine could enter through the cell surface and was immediately phosphorylated.

In this study, we tested the growth inhibition of six pyrimidine analogs (5-fluorouracil, floxuridine, 5’-deoxy-5-fluorouridine, 5-fluorouridine, gemcitabine, and cytarabine) on wild-type CU428 and conditional mutant NP1 T. thermophila. Whereas several 5-fluorouracil derivatives have been shown to inhibit Tetrahymena pyriformis growth [6, 11], more recently developed pyrimidine analogs, such as gemcitabine, a cytidine analog, and 5’-deoxy-5-fluorouridine, have not been evaluated in Tetrahymena. Furthermore, little is known about how these pyrimidine analogs are transported into and act in the cells.

Our results show that except for cytarabine, the tested pyrimidine analogs are capable of inhibiting T. thermophila growth with different half-maximal inhibitory concentrations (IC50). We also show that phagocytosis is not required for pyrimidine analog uptake. Finally, by supplementing T. thermophila with higher concentrations of exogenous nucleobases and nucleosides in the rescue experiments, some but not all inhibitory effects from the analogs could be reversed. Implications from our study and how our results conform to bioinformatic predictions based on the T. thermophila genome are also discussed [19, 20].

Materials and methods

Tetrahymena thermophila growth curve measurements

T. thermophila wild-type CU428 (Stock ID: SD00178, MPR1; mp-s, VII) and conditional mutant NP1 strains (Stock ID: SD01422) [21, 22] were acquired from the Tetrahymena Stock Center. Cells were maintained and grown at room temperature (air-conditioned at 22°C) in modified Neff’s medium (0.25% proteose peptone, 0.25% yeast extract, 0.5% glucose, and 33.3 μM FeCl3) in Erlenmeyer flasks until the exponential growth phase. The absence of the oral apparatus and phagocytosis of the NP1 strain at the restrictive temperature (37°C) was confirmed by feeding the cells Congo Red dye and examining the presence of red food vacuoles under light microscope (S1 Fig). The final cell concentration to start growth curve experiments was approximately 4,000 cells/mL.

Stock solutions of pyrimidine bases, nucleosides, and analogs were prepared by dissolving each chemical: uracil (Sigma-Aldrich, St. Louis, MO, USA), thymine (Sigma-Aldrich), uridine (Cayman Chemical, Ann Arbor, MI, USA), thymidine (Cayman Chemical), 5-methyluridine (Cayman Chemical), 5-fluorouracil (Cayman Chemical), floxuridine (Cayman Chemical), 5’-deoxy-5-fluorouridine (TCI, Portland, OR, USA), 5-fluorouridine (TCI), gemcitabine (Cayman Chemical), and cytarabine (Cayman Chemical) in modified Neff’s medium. The stock concentration for each chemical is provided in the S2 File. Paromomycin (Cayman Chemical) and cycloheximide (Sigma-Aldrich) stock solutions were prepared in d2H2O at 100 mg/mL and 10 mg/mL, respectively.

Growth inhibition and rescue experiments were conducted in triplicates at room temperature for 48 hours. In the rescue experiments, additional nucleobases (uracil and thymine) or nucleosides (uridine, thymidine, and 5’-methyluridine) were added to T. thermophila culture alongside the pyrimidine analogs in modified Neff’s medium. The final concentration of additional nucleobases and nucleosides was set at 5 mM, which was substantially higher than the concentrations of ribonucleotides present in the modified Neff’s medium from the yeast extract (4% of the dry weight, or 0.3 mM, according to [23]). Each inhibitory analog was tested at two different concentrations. The inhibitory concentrations of the analogs in the rescue experiments varied: 5-fluorouracil (0.4 mM), floxuridine (0.4 mM), 5’-deoxy-5-fluorouridine (2.5 mM), 5-fluorouridine (0.5 mM), and gemcitabine (0.04 mM). Each analog was also tested at its IC50 concentration. Analogs that might be rescued by additional nucleobases or nucleosides were further tested in an additional concentration: 5-fluorouracil (0.05 mM), floxuridine (0.08 mM), and 5’-deoxy-5-fluorouridine (1.25 mM). Paromomycin and cycloheximide were tested at a final concentration of 10 μg/mL and 5 μg/mL, respectively.

The pyrimidine analogs were each added to T. thermophila cultures four hours after cells were incubated at 37°C to determine whether phagocytosis is required for analog entry. Cell densities were measured 24 hours after cell inoculation (S2 Fig) using either a WPA CO8000 Cell Density Meter (Biochrom, Cambridge, UK) or a Multiskan FC microplate reader with a 594 nm filter (Thermofisher, Waltham, MA, USA).

Calculation of IC50 values

Data analysis was performed on GraphPad Prism v9.2.0 (San Diego, CA, USA). Half-maximal inhibitory concentrations (IC50) of the tested pyrimidine analogs for CU428 and NP1 were calculated using a nonlinear regression analysis of log(inhibitor) vs. response ‐ variable slope (four parameters). IC50 values for each analog and their standard errors are reported in Fig 1. Independent-groups t-tests were performed to examine the statistical significance between IC50 concentrations of the same analog on CU428 and NP1. The minimal data set used for calculations and figure creation can be found in the S1 File.

Fig 1. Pyrimidine analog structures and calculated IC50 values for CU428 and NP1 Tetrahymena thermophila at room temperature (RT) and 37°C.

Results and discussion

IC50 values of pyrimidine analogs inhibiting CU428 and NP1 growth

Of the six tested pyrimidine analogs, cytarabine (up to 0.5 mM) was the only compound that did not inhibit T. thermophila growth (Fig 1). This is consistent with results reported in T. pyriformis, where no growth inhibition was observed with 0.22 mM of cytarabine (arabinosylcytosine) [6]. The other five analogs inhibited CU428 and NP1 cell growth with IC50 values ranging from the low single digits to hundreds of micromolar, with the lowest being 1.61 μM (gemcitabine) and the highest being 654.20 μM (5’-deoxy-5-fluorouridine, Fig 1). Independent-groups t-tests comparing each analog’s IC50 values for CU428 and NP1 at room temperature found no significant differences for any of the five analogs (p > 0.3, S1 Table).

Our findings are consistent with results obtained using T. pyriformis grown in proteose-peptone medium [6], demonstrating that T. thermophila growth can be inhibited by 5-fluorouracil, 5-fluorouridine, and floxuridine (5-fluorodeoxyuridine). In addition, we found that both gemcitabine and 5’-deoxy-5-fluorouridine are effective in inhibiting the growth of T. thermophila. Gemcitabine was recently shown to inhibit the growth of the marine ciliate Euplotes vannus [24], but has not been tested using other ciliate species.

The uptake of pyrimidine bases and nucleosides in T. thermophila does not require phagocytosis

To test whether the uptake of pyrimidine analogs depends on phagocytosis, we cultured both CU428 and NP1 cells at 37°C, a temperature where the conditional mutant NP1 does not form the oral apparatus required for phagocytosis [21]. The absence of phagosomes was confirmed by using a Congo Red stain before the addition of pyrimidine analogs (S1 Fig). All five analogs inhibited cell growth of both the wild-type CU428 and the ‘mouthless’ NP1 cells at 37°C (Fig 1), which strongly suggests that the uptake of pyrimidine analogs, including nucleobases and nucleosides in T. thermophila, does not require an oral apparatus and phagocytosis.

Specifically how nucleobases and nucleosides are transported into Tetrahymena remains unknown. Several early studies in T. pyriformis suggested that the transport of these compounds might involve a nucleoside transporter system [16, 25, 26]. Transporter proteins in this system have the ability to move one or more types of nucleotides across membranes, with functional redundancies [16, 26]. Indeed, results derived from recent bioinformatic analyses suggest that there are more than 10 equilibrative nucleoside transporter genes present in the T. thermophila genome [27, 28], indirectly confirming those early observations. However, the functions and locations of these putative transporters, as well as the mode of transport, require further investigation.

No significant differences were found between each analog’s IC50 values for CU428 and NP1 at 37°C (all p > 0.2).The IC50 values of 5-fluorouracil, 5-fluorouridine, and floxuridine on CU428 and NP1 did not change significantly between room temperature and 37°C (all p > 0.1). In contrast, IC50 values calculated from growth at 37°C of 5’-deoxy-5-fluorouridine decreased significantly (p = .001) while that of gemcitabine increased significantly (p = .011), when comparing to those values measured at room temperature (Fig 1). We do not have a simple explanation for the changes (or lack thereof) in IC50 values, as the exact inhibition mechanisms for each of the analogs in T. thermophila remain unclear. An analog may act on one or more enzymes, and may have varying binding affinities to enzymes at different temperatures.

Growth inhibitory effects may be reversed by adding additional nucleobases or nucleosides

To further investigate how the five pyrimidine analogs might affect pyrimidine metabolism in T. thermophila, we added additional pyrimidine bases and nucleosides to determine whether they could compete against the analogs and reverse the growth inhibition. Two pyrimidine bases (uracil and thymine) and three nucleosides (uridine, thymidine, and 5-methyluridine) were used in the rescue experiments; the nucleobases and nucleosides were added alongside the pyrimidine analogs to T. thermophila culture. We expected that higher concentrations of uracil and uridine might outcompete 5-fluorouracil and its derivatives, but not gemcitabine, which is a cytidine analog. Furthermore, if thymidylate synthase, which synthesizes thymidine for DNA replication by converting dUMP to dTMP, is the primary inhibitory site for 5-fluorouracil and its derivatives as shown in human cancer cells [29, 30], supplying T. thermophila with additional thymine or thymidine should mitigate the inhibitory effect (Fig 2). We did not expect 5-methyluridine to reverse the inhibitory effects since it is inert in supporting the growth of T. pyriformis [1113]. In addition, two antibiotics, paromomycin and cycloheximide, that inhibit the growth of Tetrahymena by blocking protein synthesis [31, 32], are not expected to be rescued from any of the supplementary pyrimidine bases or nucleosides.

Fig 2. Schematic drawing of uracil and uridine pathways in T. thermophila.

The question mark indicates an absence of a uridine phosphorylase homolog interconverting uridine and uracil through bioinformatic searches, but such enzyme activity was strongly suggested by biochemical experiments (see main text).

In the rescue experiments, the concentration of the five rescuing pyrimidine bases and nucleosides was fixed at 5 mM. This concentration was substantially higher than the two concentrations of each pyrimidine analog tested: an inhibitory concentration and the IC50 (see Materials and Methods). For 5-fluorouracil, floxuridine, and 5’-deoxy-5-fluorouridine, whose inhibitory effects were found to be reversible in the rescue experiments, we tested an intermediate concentration between the inhibitory concentration and the IC50.

As predicted, 5-methyluridine was not able to reverse the inhibitory effects from any of the five analogs (Figs 36) and none of the five pyrimidine bases and nucleosides we tested could reverse the inhibitory effect of gemcitabine and the two antibiotics (Fig 6A, 6B, 6E and 6F). Additionally, growth inhibition from 5-fluorouridine could not be reversed by any of the five pyrimidine bases and nucleosides (Fig 6C and 6D). In the absence of the pyrimidine analogs, none of the additional pyrimidine bases or nucleosides affected T. thermophila growth (Figs 3D, 4D and 5D).

Fig 3.

Growth curves of T. thermophila supplemented with 5 mM of uracil, uridine, thymine, thymidine, or 5-methyluridine in modified Neff’s medium containing: (A) 0.4 mM. (B) 0.05 mM. (C) 0.005 mM. (D) 0.0 mM 5-fluorouracil. Data plotted ± SEM.

Fig 4.

Growth curves of T. thermophila supplemented with 5 mM of uracil, uridine, thymine, thymidine, or 5-methyluridine in modified Neff’s medium containing: (A) 0.4 mM. (B) 0.08 mM. (C) 0.015 mM. (D) 0.0 mM floxuridine. Data plotted ± SEM.

Fig 5.

Growth curves of T. thermophila supplemented with 5 mM of uracil, uridine, thymine, thymidine, or 5-methyluridine in modified Neff’s medium containing: (A) 2.5 mM. (B) 1.25 mM. (C) 0.5 mM. (D) 0.0 mM 5’-deoxy-5-fluorouridine. Data plotted ± SEM.

Fig 6.

Growth curves of T. thermophila supplemented with 5 mM of uracil, uridine, thymine, thymidine, or 5-methyluridine in modified Neff’s medium containing: (A) 0.04 mM gemcitabine. (B) 0.002 mM gemcitabine. (C) 0.5 mM 5-fluorouridine. (D) 0.005 mM 5-fluorouridine. (E) 10 μg/mL paromomycin. (F) 5 μg/mL cycloheximide. Data plotted ± SEM.

We found that uracil but not uridine, and thymine but not thymidine, could reverse the inhibitory effects from 5-fluorouracil (Fig 3). Uracil was capable of reversing the inhibitory effect at all three 5-fluorouracil concentrations (Fig 3A–3C). The rescue effect of thymine was most profound at the intermediate concentration of 5-fluorouracil (0.05 mM, Fig 3B), but not at a higher, inhibitory concentration (0.4 mM, Fig 3A).

Growth inhibition from floxuridine could only be reversed by thymidine at the intermediate concentration and IC50 (Fig 4), and those from 5’-deoxy-5-fluorouridine could only be reversed by uracil at the two lower concentrations (Fig 5).

While early studies suggested that uracil and uridine were equally effective as the sole pyrimidine source supporting Tetrahymena growth [11], our rescue experiments showed that only uracil was capable of reversing the growth blockages in T. thermophila caused by 5-fluorouracil and its prodrug 5’-deoxy-5-fluorouridine (Figs 3 and 5). This finding suggests that uracil and uridine are not as interchangeable in Tetrahymena as once thought. One clue for this non-interchangeability may be found in the Tetrahymena genome [20] as the key enzyme responsible for interconverting uridine and uracil is absent, according to the KEGG pathway (S3 Fig) [19]. However, results derived from several early experiments strongly suggest the presence of uridine phosphorylase activity in T. pyriformis [3335]. Whether these discrepancies indicate the presence of a non-canonical uridine phosphorylase in Tetrahymena warrants further investigation. Our results suggest that uridine and uracil are differentially metabolized in T. thermophila: uridine may be preferentially used in RNA synthesis and uracil for DNA synthesis when both are available. Because 5-fluorouracil and 5’-deoxy-5-fluorouridine primarily target thymidylate synthase for dTMP synthesis (EC in S3 Fig, review in [36]), uracil may be more effective in reversing their inhibitory effects.

Interestingly, in addition to uracil and uridine, thymine and thymidine also appear to be differentially metabolized in Tetrahymena. This was first indicated in an early experiment where thymidine, but not thymine, was shown to reinitiate growth of T. pyriformis grown in defined medium lacking folic acid [11]. We show that thymine, but not thymidine, reversed the growth blockage from 5’-deoxy-5-fluorouridine in T. thermophila grown in modified Neff’s medium (Fig 5). Thymidine, but not thymine, could similarly reverse the growth blockage from floxuridine (Fig 4). Altogether, these observations strongly suggest inefficient interconversions between thymine and thymidine, despite the biochemical and bioinformatic detection of thymidine phosphorylase activity in Tetrahymena (S3 Fig, EC [37]. Indeed, it has been shown that thymidine is a much more effective precursor than thymine to be incorporated into DNA in Tetrahymena among other eukaryotic cells [38, 39]. Finally, the fact that thymine, but not thymidine, could reverse the growth blockage from 5’-deoxy-5-fluorouridine indicates that this nucleobase might be able to be converted to TMP without going through a thymidine intermediate. However, other mechanisms (e.g., competitive or allosteric regulation) to interpret our results remain plausible.

It is noteworthy that our results show that thymidine, but not uridine, rescued the growth of T. thermophila from floxuridine blockage in modified Neff’s medium (Fig 4). This would contradict the results from T. pyriformis grown in a defined medium that uridine, but not thymidine, reversed the growth blockage from the same analog [11]. It is not clear what may have caused the discrepancies. The concentrations of floxuridine used in both studies were similar while our experiments used a much higher concentration of rescuing pyrimidines (5 mM) compared to approximately 41 μM (10 mg/mL) in Wykes and Prescott’s study [11]. In addition, the Tetrahymena species and growth media used in the two studies were different. More experiments are clearly needed to further elucidate the blockage mechanisms of floxuridine and other pyrimidine analogs and the mechanism of nucleotide transport and metabolism in different Tetrahymena species.


In this study, we demonstrated that 5-fluorouracil, floxuridine, and 5-fluorouridine inhibited the growth of T. thermophila grown in modified Neff medium (compared to T. pyriformis in a defined medium in previous studies). Furthermore, we showed that both gemcitabine and 5’-deoxy-5-fluorouridine inhibit T. thermophila growth, while cytarabine does not. We present new evidence suggesting that these pyrimidine analogs may enter T. thermophila through their peripheral membrane systems. Results obtained from the rescue experiments suggest differential metabolism of uracil and uridine, and thymine and thymidine in T. thermophila.

Supporting information

S1 Fig. Micrographs of CU428 and NP1 T. thermophila fed Congo Red dye at room temperature (RT) and 37°C.

Micrographs taken at 20X magnification, 1.5X zoom. (A) CU428 at RT. (B) NP1 at RT. (C) 428 at 37°C. (D) NP1 at 37°C. The absence of dark vacuoles in NP1 at 37°C indicates a nonfunctional oral apparatus.


S2 Fig. T. thermophila growth (O.D.) as a function of increasing pyrimidine analog concentrations.

Data plotted ± SEM.


S3 Fig. KEGG pyrimidine metabolism pathway of T. thermophila.

Homologous genes detected in the T. thermophila genome are highlighted in green. This diagram was created and downloaded from the KEGG database [19] by selecting the pathway type for T. thermophila.


S1 Table. Comparisons of IC50 values of pyrimidine analogs on CU428 and NP1 at room temperature and 37°C.


S1 File. The minimal data set used for calculations and figure creation.


S2 File. Supplementary materials and methods.



The authors would like to thank Drs. Thomas Doak, F. Paul Doerder, Donna Cassidy-Hanley, and Ed Orias for providing background information and guidance on Tetrahymena strains.


  1. 1. Brownell JE, Zhou J, Ranalli T, Kobayashi R, Edmondson DG, Roth SY, et al. Tetrahymena histone acetyltransferase A: a homolog to yeast Gcn5p linking histone acetylation to gene activation. Cell. 1996;84: 843–851. pmid:8601308
  2. 2. Kruger K, Grabowski PJ, Zaug AJ, Sands J, Gottschling DE, Cech TR. Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena. Cell. 1982;31: 147–157. pmid:6297745
  3. 3. Greider CW, Blackburn EH. Identification of a specific telomere terminal transferase activity in Tetrahymena extracts. Cell. 1985;43: 405–413. pmid:3907856
  4. 4. Sauvant MP, Pepin D, Piccinni E. Tetrahymena pyriformis: a tool for toxicological studies. A review. Chemosphere. 1999;38: 1631–1669. pmid:10070737
  5. 5. Maurya R, Pandey AK. Importance of protozoa Tetrahymena in toxicological studies: A review. Sci Total Environ. 2020;741: 140058. pmid:32599397
  6. 6. Hill DL, Straight S, Allan PW. Use of Tetrahymena pyriformis to evaluate the effects of purine and pyrimidine analogs. J Protozool. 1970;17: 619–623. pmid:5505363
  7. 7. Stargell LA, Heruth DP, Gaertig J, Gorovsky MA. Drugs affecting microtubule dynamics increase alpha-tubulin mRNA accumulation via transcription in Tetrahymena thermophila. Mol Cell Biol. 1992;12: 1443–1450. pmid:1347905
  8. 8. Hill DL. Chapter 6 ‐ Purine, Pyrimidine, and Nucleic Acid Metabolism. In: Hill DL, editor. The Biochemistry and Physiology of Tetrahymena. Academic Press; 1972. pp. 125–162.
  9. 9. Kidder GW, Dewey VC. Studies on the Biochemistry of Tetrahymena. XIV. The Activity of Natural Purines and Pyrimidines. Proc Natl Acad Sci U S A. 1948;34: 566–574.
  10. 10. Kidder GW, Dewey VC. Studies on the biochemistry of Tetrahymena V. The chemical nature of factors I and III. Arch Biochem. 1945;8: 293.
  11. 11. Wykes JR, Prescott DM. The metabolism of pyrimidine compounds by Tetrahymena pyriformis. J Cell Physiol. 1968;72: 173–183. pmid:5724568
  12. 12. Kidder GW, Dewey VC. The biological activity of substituted pyrimidines. J Biol Chem. 1949;178: 383–387. pmid:18112121
  13. 13. Kusama K, Prescott DM, Fröholm LO, Cohn WE. The formation and metabolic interchanges of pseudouridine in Tetrahymena pyriformis. J Biol Chem. 1966;241: 4086–4091. pmid:5920814
  14. 14. Rasmussen L. Nutrient uptake in Tetrahymena pyriformis. Carlsberg Res Commun. 1976;41: 143–167.
  15. 15. Nusblat AD, Bright LJ, Turkewitz AP. Conservation and innovation in Tetrahymena membrane traffic: proteins, lipids, and compartments. Methods Cell Biol. 2012;109: 141–175. pmid:22444145
  16. 16. Freeman M, Moner JG. The uptake of pyrimidine nucleosides in Tetrahymena. I. Uridine. J Protozool. 1976;23: 465–472. pmid:823331
  17. 17. Rasmussen L, Orias E. Tetrahymena: Growth Without Phagocytosis. Science. 1975;190: 464–465. pmid:1166313
  18. 18. Rasmussen L. On the role of food vacuole formation in the uptake of dissolved nutrients by Tetrahymena. Exp Cell Res. 1973;82: 192–196. pmid:4356487
  19. 19. Kanehisa M, Goto S. KEGG: kyoto encyclopedia of genes and genomes. Nucleic Acids Res. 2000;28: 27–30. pmid:10592173
  20. 20. Eisen JA, Coyne RS, Wu M, Wu D, Thiagarajan M, Wortman JR, et al. Macronuclear genome sequence of the ciliate Tetrahymena thermophila, a model eukaryote. PLoS Biol. 2006;4: e286. pmid:16933976
  21. 21. Orias E, Pollock NA. Heat-sensitive development of the phagocytotic organelle in a Tetrahymena mutant. Exp Cell Res. 1975;90: 345–356. pmid:803447
  22. 22. Silberstein GB, Orias E, Pollock NA. Mutant with heat-sensitive capacity for phagocytosis in tetrahymena: isolation and genetic characterization. Genet Res. 1975;26: 11–19. pmid:1218733
  23. 23. Tao Z, Yuan H, Liu M, Liu Q, Zhang S, Liu H, et al. Yeast Extract: Characteristics, Production, Applications and Future Perspectives. J Microbiol Biotechnol. 2023;33: 151–166. pmid:36474327
  24. 24. Wang Q, Xu H, Warren A. A Bioassay for the Cytotoxicity of Gemcitabine Using the Marine Ciliate Euplotes vannus. Journal of Ocean University of China. 2019;18: 675–679.
  25. 25. Wolfe J. Uridine uptake in a unicellular eukaryote during the interdivision period and after growth arrest. J Cell Physiol. 1975;85: 73–85. pmid:803272
  26. 26. Matsuura T, Ueyama H, Ueda K. Transport of Thymidine and Uridine in Tetrahymena pyriformis. Cell Struct Funct. 1976;1: 313.
  27. 27. Kumar U, Saier MHJ. Comparative genomic analysis of integral membrane transport proteins in ciliates. J Eukaryot Microbiol. 2015;62: 167–187. pmid:25099884
  28. 28. Elbourne LDH, Tetu SG, Hassan KA, Paulsen IT. TransportDB 2.0: a database for exploring membrane transporters in sequenced genomes from all domains of life. Nucleic Acids Res. 2017;45: D320–D324. pmid:27899676
  29. 29. Kono A, Hara Y, Sugata S, Karube Y, Matsushima Y, Ishitsuka H. Activation of 5’-Deoxy-5-fluorouridine by thymidine phosphorylase in human tumors. Chem Pharm Bull. 1983;31: 175–178. pmid:6221809
  30. 30. Longley DB, Harkin DP, Johnston PG. 5-Fluorouracil: Mechanisms of action and clinical strategies. Nat Rev Cancer. 2003;3: 330–338. pmid:12724731
  31. 31. Wang TC, Hooper AB. Adaptation to cycloheximide of macromolecular-synthesis in Tetrahymena. J Cell Physiol. 1978;95: 1–11. pmid:417087
  32. 32. Palmer E, Wilhelm JM. Mistranslation in a eukaryotic organism. Cell. 1978;13: 329–334. pmid:75070
  33. 33. Hill DL, Chambers P. The purine and pyrimidine metabolism of Tetrahymena pyriformis. J Cell Physiol. 1967;69: 321–329. pmid:5584324
  34. 34. Conner RL, Linden C. Purine and pyrimidine 5′-nucleotide catabolism in Tetrahymena pyriformis W1. J Protozool. 1970;17: 659–662. pmid:5505364
  35. 35. Eichel HJ. Some pyrimidine-metabolizing enzymes of Tetrahymena pyriformis. J. Protozool. 1957;4: 209–220.
  36. 36. Ghafouri-Fard S, Abak A, Tondro Anamag F, Shoorei H, Fattahi F, Javadinia SA, et al. 5-Fluorouracil: A narrative review on the role of regulatory mechanisms in driving resistance to this chemotherapeutic agent. Frontiers in Oncology. 2021;11: 658636. pmid:33954114
  37. 37. Purine Friedkin M. and pyrimidine metabolism in microorganisms. J Cell Comp Physiol. 1953;41: 261–282. pmid:13052640
  38. 38. Friedkin M, Wood HIV. Utilization of thymidine-C14 by bone marrow cells and isolated thymus nuclei. J Biol Chem. 1956;220: 639–651. pmid:13331922
  39. 39. Heinrich MR, Dewey VC, Kidder GW. The origin of thymine and cytosine in Tetrahymena. Biochim Biophys Acta. 1957;25: 199–200. pmid:13445743